In general, a roller type one-way clutch is composed primarily of an outer race in which at least one pocket having a cam surface on an inner periphery thereof is formed, an inner race which is disposed concentrically with the outer race and which has an outer peripheral track surface, a roller which is disposed in the pocket and which transmits a torque between the outer peripheral track surface of the inner race and the inner peripheral cam surface of the outer race, and a spring which is in contact with the idling side of the roller.
In the roller type one-way clutch having the configuration described above, the inner race is rotated only in one direction with respect to the outer race by a cam mechanism composed of a roller and a cam surface. More specifically, the inner race is configured to idle in one direction with respect to the outer race and impart a rotational torque to the outer race through the intermediary of the cam mechanism only in the opposite direction.
For example, a one-way clutch used with a starter for a two-wheel vehicle is subjected to severe vibration due to high revolutions and also to heavy dust. Hence, foreign substances, such as wastes containing abrasion powder, tend to accumulate in the one-way clutch. The foreign substances in the one-way clutch adversely affect the engaging performance, so that it is desirable to promptly remove the foreign substances in order to improve the reliability of the one-way clutch.
Meanwhile, there is a roller type one-way clutch used with the starter of a two-wheel vehicle, as the one described in Patent Document 1, in which a side plate having a spline groove which is provided in the inside diameter portion thereof and with which a member, such as an engine crankshaft, engages, is bolted to the outer race of the one-way clutch.
Further, Patent Document 2 describes a side plate provided with a foreign substance discharge hole through which foreign substances are discharged to the outside. The annular side plate is secured to an end surface in the axial direction of an annular outer race which has substantially the same diameter as that of the side plate.